Layered structure of the Earth

Fig. 4.3 . The Earth's internal structure is determined by the varying velocity of earthquake waves. There are two types of waves that travel through the Earth. They are known as the compression P, or push and pull, waves and the shear S, or shake, waves. The P waves move almost twice as fast as the S waves, and the P waves pass through the fluid outer core, which the S waves cannot do. The boundary between the mantle and core is marked by a precipitous drop in the velocity of the P waves at a depth of about 2.9 thousand kilometers. The S waves do not propagate beyond this boundary. The liquid outer core is separated from the solid inner core at a radius of 1.22 kilometers where the P waves increase in velocity.